Generally, there are two techniques employed for measuring the breath alcohol concentration and thereby determine a person's blood alcohol concentration. In a first method, infrared spectroscopy is used, whereby a breath sample from a person is subjected to infrared radiation. The molecules in the breath sample absorb specific frequencies, called resonant frequencies, which are characteristic to the molecules. For example the absorption by ethanol molecules gives rise to a specific infrared spectrum which may be used to determine the amount of ethanol present in the breath sample, and thus the breath alcohol concentration. Although this method gives high measuring accuracy, sensors incorporating infrared spectroscopy are expensive, which limits application in mass-produced devices.
A second commonly used technology is based on a fuel cell sensor which converts fuel in the shape of alcohol (ethanol) to electric current in an electrochemical reaction. Fuel cell sensors have a somewhat lower accuracy than infrared spectroscopy sensors, but are much cheaper. However, fuel cell sensors require that the breath sample is of a determinable volume in order to correctly determine the breath alcohol concentration.
Traditional fuel cell based analyser systems operate by means of a mechanical sampling system which draws a pre-specified volume of breath into the fuel cell for analysis. The mechanical means may comprise motors, solenoid valves, piston-cylinder devices, diaphragm mechanisms or push buttons connected to a pump or bellows system. In U.S. Pat. No. 6,167,746 there is disclosed an apparatus comprising an electronically controlled valve to ascertain that a requisite volume of breath is passed through a fuel cell. US 2005/0241871 discloses a sobriety interlock device comprising a pressure transducer and a solenoid valve operating independently of each other providing a variable flow of breath to a fuel cell. A microprocessor instructs the solenoid valve to remain open for a finite period of time to give a predetermined breath sample volume, and calculates an algorithmic correction factor based on pressure readings to provide a pressure compensated alcohol result.
The methods described in the prior art involve advanced control circuitry and complex or bulky mechanical components which introduce extra cost to the system and limit the ability to reduce the size of the system without compromising accuracy.
International application PCT/SE2010/051421, belonging to the applicant, discloses a method and apparatus for measuring breath alcohol concentration overcoming many of the problems associated with the prior art. However, the design of the mouthpiece of the apparatus has shown that there is a non-linear relationship between the flow rate and the final reading. In other words, varying flow rate gives different measurements of breath alcohol concentration, even for similar or identical alcohol concentrations.
Hence, there is a need for improved methods for measuring breath alcohol concentration with high accuracy, which allow for compact devices that may be produced at low cost.